1986, Challenger, Disaster, ET, External Tank, January 28, NASA, Solid Rocket Boosters, Space Shuttle, SRB, STS-51L
Many experts have discussed what happened in the moments up to and after the breakup of the Challenger Space Shuttle on January 28, 1986. NASA thoroughly investigated the events the led to the loss of the vehicle and the seven astronauts on board. This information was released over the months (and years) following the disaster, but here is a synopsis of what has been learned and discussed.
What Didn’t Happen
Not An Explosion
The Challenger and the External Tank (ET) did not ‘explode’ in the sense of a violent, pressure wave of energy. The fireball that engulfed the Space Shuttle was triggered when the bottom of the External Tank broke away releasing all the hydrogen fuel. This fuel ignited and gave the ET a sudden acceleration or upward push, which then caused the rupture of the oxygen tank in the upper portion of the External Tank that tore off the top of the ET¹. The result of the escaping fuel from the top of the ET created an oxygen-rich environment around the vehicle. The fuel in the Orbiter for the thrusters also ignited which may have been released when the nose of the Challenger sheared off due to aerodynamic forces.
(¹There is also evidence that the nose of the starboard Solid Rocket Booster swung into the ET and contributed to the rupture at the top of the tank.)
The failure at both the bottom and top of the External Tank ultimately led to its breakup because it no longer had an aerodynamic structure to force the air around it. In the last images of the Space Shuttle before breakup, the entire vehicle is masked by a translucent white and gold curtain of smoke and burning fuel. The fireball that surrounded the Space Shuttle was a combination of all the liquid fuel being released and igniting.
Similarly, the breakup of the Orbiter was not caused by explosive forces from the fireball. As the ET accelerated and broke apart Challenger began pivot, nose down, so that the upper portion of the Orbiter turned into the oncoming rush of air. Since it was not aerodynamically designed to fly into that position (the Shuttle was traveling at 1,450 mph) the nose portion, including the crew compartment sheared away from the rest of the vehicle. As the crew compartment separated from the rest of the Orbiter, air rushed in to the Payload Bay and other cavities literally blowing Challenger apart from the inside.
Cold Weather and O-ring Failure Not the Entire Cause of Disaster
The joint on the starboard Solid Rocket Booster (SRB) failed and allowed extremely hot gases to be directed at the External Tank; however, the infamous O-rings (a primary and secondary for each joint) and the cold weather were probably NOT the only factors leading to the disaster.
Both Solid Rocket Boosters were subjected to subfreezing temperatures overnight (it was only 36°F at launch.) If temperature was the only factor, then other joint breaches should have occurred in the multiple joints of both SRBs. There was only one breach on one SRB and at one point in the 360° circumference around the joints.
Photographic evidence shows that a breach of the joint occurred as Challenger lifted off, but it seemed to re-seal (probably with soot and debris) as the vehicle cleared the tower. However, Challenger hit the strongest wind shear conditions of any Shuttle in twenty-five missions soon after launch. Whatever was sealing the earlier breach in the O-ring probably broke free as the SRB joints flexed in the wind shear. That started the hot gases to burn an ever-increasing hole through the joint, which was aimed at the strut that attached the External Tank to the Solid Rocket Booster.
Joint design, wind shear, O-rings, weather, and possibly an unknown factor (undetected pre-launch damage or weakness at one point of the joint) all seem to create a set of extraordinary circumstances that doomed Challenger.
T Minus 12:00:00.000 Hours
In the twelve hours before the launch, Launch Pad 39B experienced colder temperatures than had occurred prior to any Shuttle launch. This likely caused the O-rings in the joint of the SRBs to contract slightly.
At launch, possibly due to the cold weather or other causes, the joint was not completely sealed and hot gases burned through one point on a joint on the starboard Solid Rocket Booster. As Challenger lifted off puffs of black smoke appeared near the joint area 3 times per second.
+00:00:02.733 Joint Re-Seals
The black puffs of smoke are no longer visible. It is believed that debris from the O-rings temporarily re-sealed the joint.
+00:00:19.000 Wind Comment
Pilot Michael Smith says, “Looks like we’ve got a lot of wind here today.”
+00:00:36.990 Shuttle Responds to Wind Shear
Challenger automatically responds to heavy wind shear. This causes stress and flexing of the joints in the SRBs. The belief is that at some point the temporary seal formed soon after launch breaks free and hot gases begin to blow through the gap. The hole in the joint grows as the gases melt the structure around it.
+00:00:58.788 Heat Plume
Cameras record an abnormal plume of flame and smoke coming from the starboard Solid Rocket Booster. The plume grows in size over the next several seconds. The plume is aimed at the strut on the External Tank which connects it to the Solid Rocket Booster.
+00:00:60.004 Pressure Drop in SRB
Computer data shows a pressure drop in the starboard Solid Rocket Booster. While Mission Control and the crew are not fully aware of this, there is no doubt that the leak in the joint is effecting the power output of the Solid Rocket Booster. Had the flame been pointed away from the Shuttle and the External Tank, the Solid Rocket Booster would have eventually caused an abort due to lack of thrust to make orbit.
+00:00:64.660 ET Burn Through
The plume between the Solid Rocket Booster and the External Tank suddenly changes shape. This indicates that the External Tank has burned through and hydrogen fuel is leaking and increasing the flame.
+00:00:66.764 Pressure Drop in ET
Pressure in the External Tank begins to drop indicating a massive leak; however, even if the astronauts had noted the drop in pressure there was no action they could have taken. In seven seconds the entire vehicle will be engulfed in flame and the External Tank and Orbiter will be breaking up.
+00:00:70.000 “Go at throttle up.”
Commander Scobee calmly responds to Mission Controls authorization to increase speed by saying, “Roger, go at throttle up.” While events around the Solid Rocket Booster and External Tank are beginning to impact the vehicles flight path, no one on the ground or in the air has any forewarning of what is about to happen.
+00:00:72.204 Wild Nozzle Movements
At this point the engines on the Solid Rocket Boosters are shifting positions to compensate for flight path variations caused by the cascading failures of the ET and SRB. These movements of the engines on the SRB and then by the Main Engines on the Orbiter become wilder over the next second. The computer is desperately attempting to keep the vehicle on the flight path.
+00:00:73.124 Beginning of the End
Challenger is traveling at almost twice the speed of sound. Seventy-three (73) seconds into the flight the lower strut on the External Tank, which has been the target of the blow torch of hot gases leaking from the SRB, gives way and the lower end of the starboard Solid Rocket Booster flies free. It begins to pivot around the upper support. At about the same time the bottom of the External Tank comes off allowing all the hydrogen to escape and ignite. This causes a rapid acceleration of the External Tank. The upward pressure on the interior of the ET then causes the a rupture at the top of the tank, which then releases the oxygen around the Shuttle. In rapid succession, the External Tank breaks up, the Solid Rocket Boosters completely separate from the vehicle, and the Shuttle is pushed into a pivot that causes the nose to shear off at the point just in front of the Shuttle bay.
The Next 207 Seconds
The crew compartment is violently thrown around, but the G-forces most likely are not severe enough to seriously injure the crew. Ultimately, the crew cabin continues to move up from approximately 46,000 feet to over 60,000 feet, until its forward momentum is lost and it begins to arc down to the ocean below. At one point the crew probably experience weightlessness as the cabin begins to fall. The crew compartment begins a rotation that will continue until about 100 seconds before impact when it seems to stabilize with the black tiles on the bottom of Challenger’s nose facing the shore.
There is little doubt that most, if not all, of the seven astronauts survived the breakup. Personal Egress Air Packs (PEAP) were activated by the crew and switches at the pilot’s station were changed from launch position. In both cases, the break up of the vehicle, nor the impact on the ocean could have caused these actions. It is believed that all astronauts were strapped into their seats at the time of impact, which would be expected regardless of their state of consciousness.
The oxygen supply for the crew was behind them and lost in the break up. It is likely that the crew knew they lost their oxygen supply, possibly due to a loss in cabin pressure and were seeking to reestablish oxygen flow via the PEAPs. Three of four air packs were activated, unfortunately, if cabin pressure was lost the air packs would not have offered pressurized flow and therefore the crew would have lost consciousness. How quickly that would have happened would have depended on the speed of the possible decompression of the crew compartment.
Regardless of their condition during free fall, the crew would have been killed instantly upon impact in the Atlantic Ocean at 207 seconds after the break up of Challenger. The upper left section of the compartment was likely the point of first impact. It would be over six weeks before the remains were found and recovered.
SD Dude said:
The Challenger crew cabin fell into the Atlantic, not the Pacific.
Paul Kiser said:
Thanks for reading this article and commenting on the error. I’ve corrected it. No excuse, it was just a failure in catching it during my rewrites.
Jerry Gregg said:
Send me an email and I can give you the real root cause of the 51L accident. Hint, it would have occured had they scrubbed that day and launched at 68 degrees. The root cause was not cold but an assembly issue.